A susceptor is a mechanical part that functions as a ground electrode and holds a substrate in a processing chamber during fabrication, such as plasma-enhanced chemical vapor deposition (PECVD). The susceptor includes a substrate support plate mounted on a stem, along with a lift assembly for raising and lowering the substrate within the processing chamber. The substrate is held essentially flat to facilitate the deposition process.
The extent to which the substrate is held flat generally leads to more uniform structural parameters across the substrate surface. For example, it is easier to grow a film of uniform thickness on a flat substrate than on one that may have a degree of curvature due to, for example, thermal stress. Thus, if uniform structural parameters are required by the process, the substrate must be held essentially flat.
In the absence of mechanisms which physically hold the substrate flat, substrates tend to become slightly curved during processing for a number of reasons. For example, a nonuniform temperature across the substrate tends to induce a curvature due to different amounts of thermal expansion at different areas within the substrate. In a large substrate, for example 550.times.650 square millimeters (mm.sup.2), a significant difference in thermal expansion may occur because the substrate heater may not be able to provide a uniform temperature across the large dimensions of the substrate. Further, the perimeter of the substrate has more surface area than the central areas and thus radiates heat faster than the central areas, again leading to temperature nonuniformity, thermal stress and consequent curvature. In smaller substrates, for example, substrates around 360.times.450 mm.sup.2, the problem is less pronounced but nevertheless evident.
All of the above difficulties become worse as the processing temperature rises. At a typical processing temperature of 320 degrees Celsius (.degree. C.), which is common for a glass substrate, the glass substrate will lose its flatness due to the kinds of thermal stress mentioned above.
It is thus important to hold substrates essentially flat to prevent such curvature. Previous methods and apparatuses for holding substrates flat employ a frame which physically contacts the substrate around the substrate's perimeter and holds the substrate against the substrate support by the support's weight. Several difficulties have been noticed with such systems.
First, the substrate area covered by the frame is sacrificed. Thus, there is less surface area of the substrate which may be used for devices or deposition. If the entirety of the perimeter of the substrate is under the frame, substantial loss of surface area may result.
Second, the thickness of the deposited materials is not constant near the frame. This is primarily a geometric effect and occurs because of the thickness of the frame. In regions near the center of the substrate, impinging deposition gas molecules or atoms strike the surface of the substrate over a solid angle of 2.pi. steradians corresponding to a hemisphere. Near the perimeter of the substrate, the frame partially blocks gas molecules over a significant fraction of the 2.pi. angle. Near a corner of the frame, blockage is even worse. Thus, it is expected that less gas molecules strike the substrate near the substrate's perimeter. As a result, the thickness of deposition is usually not uniform near the perimeter of the substrate.
Third, deposited material may seep under the frame. Such material cannot be used in films because its thickness is uncontrollable. This problem arises because the frame typically does not contact the substrate in an abrupt manner. In other words, the effective deposition "shadow" of the frame (the point at which edge of the frame starts to inhibit deposition) is not at the same point where the frame physically touches the substrate. One reason for this is that the frame may not be completely parallel to the substrate when intimate contact is made. As a result, some deposition may occur on the substrate under the frame. Of course, the amount of such deposition is less than on the unframed central region of the substrate. This deposition may be problematic in the sense that it is uncontrollable.
Fourth, a physical frame for holding the substrate flat constitutes a large structure to be placed in a processing chamber. As such, it is a potential source for contaminant particles in the chamber which may degrade the quality of the deposited film. This may be particularly true as the contact between the frame and the substrate often causes particle release due to friction. Such particles can also adversely affect the quality of the chamber vacuum.
Fifth, a physical frame affects the reliability of transfer when a substrate is processed in one chamber and then moved to another for further processing. In particular, as a substrate is transferred from one chamber to another, a new frame is usually used. Each frame must be aligned in each processing chamber to the same position to avoid a loss of substrate processing area due to misalignment. When misalignment occurs, some of the substrate processing area used in one chamber is shadowed by the frame in the next chamber. Further, some of the substrate previously shadowed by a frame in the one chamber is not covered in the next chamber. In both cases, these areas must be sacrificed as not having been fully processed. To combat this problem, complicated realignment mechanisms must be used to ensure the same area is covered by each frame. Such mechanisms again lead to more particle-releasing surface area in the chamber and ensuing particle contamination and breakdown. Such mechanisms are also expensive and complex, increasing markedly the manufacturing cost of the processing chamber.
The inventors have discovered a need to provide a method and apparatus for keeping substrates essentially flat to increase the usable substrate area and to enhance film uniformity across this area, particularly near the edges of the substrate. The method and apparatus should not require complex mechanisms, and should not lead to contamination of the processing chamber. The present invention fulfills these needs.